This invention relates to a method for controlling the water content of a plant in order to provide a plant which is tolerant to water-related stress such as salt stress or drought stress.
Plants are continuously exposed to stress even under normal growing conditions. Such stress is caused by various factors such as salt, drought, high temperature, low temperature, strong light, air pollution, etc. From the viewpoint of agriculture, salt and drought stress cause the most serious problems. Salt stress is observed not only in areas which have a soil inherently rich in salt but also in irrigated farms. At present, more than 10% of cultivated land suffers from some degree of salt stress. To increase food production, it is desired to grow crop in land which has previously been regarded as uncultivatable.
Drought stress is caused by unseasonable weather and geographic location. In the United States, drought causes a fall in crop yield once every several years.
Therefore, it is important to find a plant which is tolerant to water-related stress.
In this regard, it is known that water channel proteins (hereinafter sometimes referred to simply as WCH proteins) seemingly have a role to play in "water flux in plants".
These plant WCH proteins are classified into plasma membrane-located types and tonoplast-located types. Some reports have already published on the genes of these proteins.
In connection with the genes of plasma membrane-located WCH proteins, there have been reported, for example, the cloning of cDNA of a turgor responsible WCH protein derived from pea [Plant Molecular Biology 15, 11-26 (1990)]; the cloning of cDNA of a desiccation responsible WCH protein derived from Arabidopsis thaliana (Plant Cell Physiology 33, 217-224); WCH gene of Arabidopsis thaliana [The Plant Journal 6, 187-199 (1994)]; WCH gene of tomato [Plant Molecular Biology 24, 539-543 (1994)]; the introduction of an antisense gene into Arabidopsis thaliana to confirm its function in the water channel [The Plant Journal 7, 87-95 (1995)]; WCH gene of common ice plant (Mesembryanthemum crystallinum) [The plant Cell 7, 1129-1142 (1995)], etc. In addition, those derived from corn, rice, kohlrabi (Brassica oleracea), soybean, barley, etc. have been registered at gene data bases, etc.
In connection with the genes of tonoplast-located WCH proteins, there have been reported, for example, one derived from tobacco [Nucleic Acids Research 18, 7449 (1990)]; one derived from common bean [The Plant Cell 2, 525-532 (1990)]; one derived from Arabidopsis thaliana [Plant Physiology 99, 561-570 (1992)], etc. In addition, those of rice, barley, soybean, radish (Raphanus sativa), white clover (Triforium repens), alfalfa (Medicago sativa), etc. have been registered at gene data bases, etc.
As described above, it has been urgently required to provide a plant which is tolerant to water-related stress. One approach to obtain such a plant is directed to controlling the water content of the plant by, for example, gene manipulation. In general, it is considered that a plant which is tolerant to water-related stress can be obtained by increasing the water content of the plant, namely, enhancing the capability of the plant to obtain water.
To obtain a water-related stress-tolerant plant in the above-mentioned manner, WO 96/00789 and Nature 379 (22) 683-684 (1996) disclose a method for obtaining a plant tolerant to water-related stress by way of controlling the water content of the plant. The method comprises introducing the gene of an enzyme for trehalose biosynthesis into the plant, thereby to induce the synthesis and accumulation of trehalose in the plant, and to eliminate the loss in water content by way of the water-retaining effect of trehalose. The disclosure of these references is incorporated herein by reference.
However, this method suffers from various problems. Namely, a certain amount of carbon fixed by photosynthesis is consumed in this method, which is disadvantageous from the viewpoint of energy consumption. In addition, there is a risk that the accumulation of trehalose might exert adverse effects on the qualities of the product and the metabolic system of the plant.
On the other hand, it has been considered that the WCH proteins described above may participate in water flux in a plant. However, no one has suggested so far that the water content of a plant may be controlled by using these proteins.